John Deere Tractors of the Series 6030 and 7030 use a system in which a charge oil pump continuously conveys hydraulic fluid against the force of gravity from an operating reservoir formed in the differential housing into an auxiliary reservoir located at a higher level, where the hydraulic fluid is charged starting from the auxiliary reservoir over a controllable high pressure pump into a hydraulic system for the operation of vehicle hydraulic units. Hydraulic fluid no longer required by vehicle units is conducted back into the operating reservoir. The vehicle hydraulic units include, in particular, a steering and braking system, as well, if necessary, agricultural implements that can be attached to the tractor and are provided with hydraulic positioning cylinders or the like. Depending on the hydraulic fluid consumption of the vehicle units, the fluid level in the operating reservoir can vary more or less significantly.
Since a pressure of several bar exists at the charge oil pump outlet in the conveying of the hydraulic fluid, this results in addition in increased demand for the capacity of the auxiliary reservoir to resist pressure. These lead to additional costs due to the associated increased costs of the configuration. If, for example, increased hydraulic fluid consumption occurs during operation of the vehicle units, there is the possibility, that the operating reservoir is completely emptied into the auxiliary reservoir due to the continued operation of the charge oil pump, so that the charge oil pump runs “dry”, or is un-lubricated during its continued operation. The latter condition may not only be detrimental to the durability of the charge oil pump, but in addition leads to an interruption of the lubrication of the differential gear in the differential housing. It is desired to have a.